Electromagnetic radiation (“radiation”) is used in numerous ways and applications including for the transmission of signals and information in communication systems. The generation of radiation at certain frequencies is more difficult and costly than for the generation at other frequencies. As more and more complex communications systems have evolved, the need for precise control of the frequency and wavelength of the radiation generated for such systems has grown. This is particularly so for fiber optic communication systems that use lasers to generate optical carrier signals. In some fiber optic systems, such as wave-division multiplexing (“WDM”) systems, which communicate information over optical fibers by pulses of laser light with multiple channels at different carrier frequencies, precise stabilization of the optical wavelengths within plus or minus 0.2 nanometers (nm) has long been required so that adjacent carrier signals do not interfere with one another. The proximity of the carrier signals in such systems is to some extent limited by the need to accommodate fluctuations in the temperature of the systems and their environments. Consequently, a need has arisen for precisely controlled and temperature invariant radiation sources of radiation of certain desired frequencies.
Because most materials change in size as they experience a change in temperature, components of a laser, including a laser resonator, typically experience a change in size as they are heated or cooled. Most materials used in the construction of laser resonators expand as they are heated, i.e., have a positive coefficient of thermal expansion. Consequently, most lasers experience a wavelength lengthening as the heat increases in the laser as it operates. The converse is true as well, namely that most lasers experience a wavelength shortening as they cool. The impact of such a change in wavelength may be significant for applications using radiation having a wavelength that is not much greater than the change in size of the resonator due to thermal expansion. For example, semiconductor diode lasers typically exhibit a wavelength lengthening of about 0.3 nanometers per degree Celsius. For a wavelength of 905 nanometers, which is a common wavelength for diode lasers, a change in temperature of one degree Celsius produces a change in output wavelength of 0.3 nanometers, which corresponds to a change in frequency of the output of the laser of approximately 1.0×1018 Hertz (Hz) or 1.0×106 tera Hertz (THz). Such thermally induced changes in frequency may be unacceptable for many applications, especially when greater changes in temperature and the correspondingly greater shifts in frequency and wavelength occur.
Maintaining the output frequency and wavelength of a single optical wavelength laser diode by cooling and/or heating means is known, see for example U.S. Pat. No. 6,229,832, which discloses an “Optical Wavelength Stability Control Apparatus, Optical Transmitter and Multiple Wavelength Transmitter.” These apparatus include one or more laser diode modules, which each have a laser diode and a photodiode to detect optical power from the laser diode. Each separate photodiode consequently diminishes the optical intensity of the system and adds to the complexity and cost.
Despite known methods for the temperature stabilization of single radiation sources, it has been particularly difficult to inexpensively produce radiation of certain frequencies and wavelengths including, for example wavelengths in the millimeter region of the spectrum, and particularly such radiation that is temperature-stabilized. Among other things, radiation in this region of the spectrum is useful in radar, radio telescope, and imaging applications.
Optical heterodyning has been seen as one way to produce millimeter wave radiation. Certain attempts have been made to couple the outputs of two or more laser sources to produce radiation at millimeter wavelengths. U.S. Pat. No. 5,007,058 discloses a “Millimeter Wave Power Generator” that combines two laser beams. The combined beam is diffracted into a plurality of externally powered, optical-to-millimeter wave transducers. A plurality of antennas is provided, one between each pair of adjoining optical-to-millimeter wave transducers. The antennas are parallel to each other, and each is driven by the optical-to-millimeter wave transducers at its ends. The back propagating millimeter wave radiation is reflected forward by a wire grid parallel to the antennas. The grid is situated between a diffractor and the optical-to-millimeter wave transducers, and is spatially tuned to constructively interfere the reflected back propagating wave with the forward propagating millimeter wave radiation. The use of a plurality of antennas and array of transducers add to the complexity and cost of the system. This arrangement is also susceptible to fluctuations in ambient temperature.
U.S. Patent Application Publication No. US2001/0014106A1 discloses an “Optical Electromagnetic Wave Generator” in which microwaves are generated by heterodyning the outputs of two or more optical lasers which have differing central frequencies to produce beat frequencies in the microwave range. One of the beat frequencies is used to modulate the output of at least one of the lasers to produce sidebands which differ from the central frequency by an integral number of the sideband frequency. Each laser is connected to one of the other lasers by a weak optical link to injection lock the laser to the sideband of the other laser. This configuration is susceptible to frequency drift of the outputs of the optical lasers arising from variations in ambient temperature.